The preparation of porous cellulose ester filter materials, including hollow cellulose ester fibers, is well known in the separations field. Such fibers are used for reverse osmosis desalination, kidney replacement dialysis machines and other hyper- or ultrafiltration processes. These fibers are essentially asymmetric membranes where either the interior or exterior surface has a dense well-defined structure or layer that severely restricts the flow of substances. The opposite surface and body of the fiber are made up of interconnecting pores which act only as a support for the dense layer and are not intended to restrict material flow in any substantial way. Usually they are made by first passing the fiber through an air stream where a dense exterior skin is formed and then into a water coagulating bath where the porous support structure is obtained. While these asymmetric membranes are very useful for various purposes, there is also a demand for symmetric porous or cellular membranes which lack this dense surface layer or skin, are at least semipermeable, and have relatively high surface area.
Kesting discloses in U.S. Pat. No. 4,035,459 the extrusion of cellulose acetate solutions with a liquid forming an interior lumen into a gas, then a coagulating bath, to form asymmetric hollow fiber cellulose acetate membranes.
Arisaka et al disclose in U.S. Pat. No. 4,127,625 the production of asymmetric hollow fibers from solutions of cellulose derivatives by extrusion of a fiber precursor, with an aqueous salt solution forming an internal cavity, directly into an aqueous coagulating bath. Compact layers can be formed on the outer and/or inner surfaces of the hollow fiber.
Joh et al disclose in U.S. Pat. Nos. 4,322,381, 4,323,627 and 4,342,711 various dry jet-wet spinning processes for producing hollow fibers of materials including cellulose esters by extruding a spinning dope from an annular slit surrounding an orifice through which other liquids are extruded to form the hollow center. The fibers are extruded so as to pass through a gas region before entering a coagulating bath which can be aqueous.
Mishiro et al disclose in U.S. Pat. No. 4,234,431 the extrusion of a dope solution of cellulose acetate, with a coagulating liquid in the center of the extrudant, into a coagulating bath which can be aqueous, to form hollow cellulose acetate fibers with a three-dimensional net-like structure of fine filtering passages forming the entire cross section of the fiber walls.
Japanese Patent Application No. 13587/1977, Japanese Patent Laid Open No. 53-99400 (or 99400/1978) discloses a fibrous tobacco filter containing 0.1 to 10 weight percent hollow fibers having an inside diameter of 40-400 microns and a "hollow percentage" (i.e., void proportion in the cross-section) of 10-70 percent. The hollow fibers can be produced of acetate materials, but nothing is disclosed of their surface properties or specific surface area. The hollow fibers are included in the tobacco filter to pass smoke essentially unfiltered during the first and second puffs, then clog with tar to divert the smoke to filtering areas on subsequent puffs.
In separation processes, it is customary to utilize hollow fibers with an asymmetric wall structure. That is, one of the fiber surfaces is different from the other in that it consists of a thin, dense skin that is selectively permeable to the desired molecular species. This is usually the outer surface. The other or inner surface should be readily permeable, with no well-defined skin character. The interior of the wall is normally cellular and porous, and serves only a support function. In the operation of separation processes, the application of elevated pressure in the system is required to achieve the desired economic mass flow.
The rate of absorption (or desorption) of a vapor from a gas stream by a column of a solid fixed absorbent is directly proportional to the surface area available per unit volume (a). This quantity is calculated as the product of the specific area of the solid and the packing density of the column and is proportional to the specific area of the solid at constant packing density.
a (1/meter)=specific surface area (sq. meter/g) x packing density (g/cu. meter)
(See for example: R.B. Bird, W.E. Steward and E.L. Lightfoot, "Transport Phenomena", Wiley, New York (1960), Chapter 22, pp. 702-705.)
In a hollow fiber for use in separation processes, it is apparent that the bulk properties of the outer layer of the wall (or other selectively permeable portion) are determinant. In contrast, in absorption (or desorption) processes, the surface properties of the walls are paramount. The wall serves as a convenient reservoir for sorbed material or material to be desorbed.
Thus, although various types of filter materials, e.g., hollow fibers, made from materials including cellulose esters are available, porous or cellular skinless hollow fibers of such materials having high surface area would be desirable products.